Numerous implant delivery systems have been devised for conveying and deploying prosthetic implants in the human body. Continued development in the field of material science has enabled an increasingly diverse range of implants to be devised which can be delivered to and implanted non-invasively at a treatment location. Such implants will typically have a low-profile delivery configuration and are expanded to a large-profile operative configuration at the treatment location. These implants can either be expanded mechanically to their operative configuration, for example by the use of balloons or pull-wires to transform the implants from their delivery configurations to their operative configuration, or they may be self-expanding implants which will naturally tend to expand to their operative configurations when released from the constraints of a delivery device.
For deployment of implants into luminal body passage-ways, catheter based delivery systems have become widely used. A typical catheter based delivery system is provided with an elongate inner shaft, with the implant mounted to the distal end portion of the shaft. An outer sheath surrounds the implant and at least the end portion of the inner shaft, to constrain the implant in its delivery configuration prior to being selectively deployed. In a typical procedure, the distal end of the delivery system is introduced percutaneously into a patient and advanced along a body lumen until the implant at the distal end portion is brought to a treatment location. The outer sheath is then removed to release the implant to be deployed at the treatment location. Because these catheter based delivery systems can be made relatively long and of a small diameter, they are particularly suited for advancement of implants to treatment locations within the human vasculature, the urinary tract and the biliary tree.
Various different arrangements have been devised for removing the outer sheath so as to release the implant for deployment at the treatment location. In one common arrangement, the outer sheath is retractable in a proximal direction relative to the inner shaft, so that the implant can be gradually released as the sheath is retracted, exposing the distal end portion of the inner shaft where the implant is mounted. These systems, however, are often relatively complex in order to provide the system with the necessary functionality. For example, the inner shaft has to be made axially substantially incompressible, so as to allow the shaft to be advanced along a body passage-way as it is pushed into the body from the proximal end. At the same time, the shaft has to be of a small diameter in order to allow it to be advanced through narrow bodily lumens, and has to be highly flexible so as to enable it to pass around the tight curvatures encountered in body passage-ways such as the human vasculature without kinking. The inner shaft also has to resist compression under the influence of the axial tensile force that has to be applied in order to retract the outer sheath. The inner shaft is often provided with a tightly wound compression spring in order to provide the necessary resistance to axial compression but the required flexibility off-axis.
The outer sheath is often responsible for providing the necessary force to constrain the implant in its delivery configuration, in particular where the implant is self-expanding and tends to exert a radially outward force trying to adopt its expanded operative configuration. Not only must the outer sheath resist these radially outward forces, but it must be retractable in a proximal direction, for which purpose it is preferably relatively inextensible in the longitudinal direction. A reinforcing structure, such as braiding, is typically used in the outer sheath to give it the necessary properties of off-axis flexibility during advancement of the catheter to the delivery location, and at the same time the necessary longitudinal strength to be proximally withdrawn for releasing the implant. The use of these reinforcements to the inner shaft and outer sheath, to give these components the necessary structural properties, thus tends to result in an increase in the thickness and overall diameter of the components and the system as a whole, which is contrary to the objective of reducing the system diameter to allow it to be advanced through narrow bodily lumens. Such systems can also be costly and time-consuming to manufacture.
An alternative arrangement for releasing the outer sheath has been devised, which can potentially reduce the axial forces associated with sheath retraction and release of the implant. The sheath is formed as a tube of material, for example ePTFE. This tube of material has the necessary strength and properties to constrain the implant in its delivery configuration. The tube is formed from a sheet of the material, folded so that opposite edges of the sheath meet and form a seam. The seam is then sewn together using a suture thread in a manner in which the thread can be released by pulling on one end of it. In use, the tube of material functions as the outer sheath during advancement of the end portion of the delivery system to a treatment location, at which point the thread is pulled to release the stitching and thus allow the tube of material to separate at the seam, facilitating deployment of the implant constrained therein. The sheet of tube material can then either be retracted along with the delivery system, after the implant has been deployed, or may be attached to the implant and left in situ at the treatment location, held between the implant and the treatment location to form part of the implant. Examples of such an implant deployment apparatus are disclosed in U.S. Pat. No. 6,352,561 B1, which is incorporated by reference in its entirety into this application, to Leopold et al., published on 5th Mar., 2002, in which the skilled reader will find examples of different stitching patterns for forming the tube of material as well as discussion of suitable materials for the outer sheath (restraining member 102) and suture or sutures (thread-like coupling member 104).
Such catheter delivery systems are particularly useful for the delivery of balloon-expandable or self-expandable stents and stent grafts, as well as valve implants and the like. With the systems employing a stitched tube of sheet material as the outer sheath, however, there can be a problem that the suture may snap before the thread is fully released from the seam of the tube, or that the stitching may become knotted or tangled at the distal end prior to complete release of the seam. This is a particular risk due to the typical length of catheter delivery systems, along which the proximally extended portion of the suture thread must pass to reach the proximal end of the delivery system where it can be manipulated, typically a meter or more in length, and the tight curvatures around which the delivery system must pass, and in which the implant may be deployed, within human bodily lumens. If the thread snaps or becomes entangled after the stitching has been only partially released, it may not be possible to retract the delivery system without damaging the bodily lumen through which the catheter has been inserted, or to fully deploy the implant. Intervention by open surgery would then be required. On the other hand, if the suture thread is made thicker, to withstand retraction forces without snapping, the overall thickness of the seam will be increased, whilst the potential for entanglement or snagging remains.
U.S. Pat. No. 6,019,785 A, which is incorporated by reference in its entirety into this application, discloses a device for delivering a prosthesis held within a sheath in a delivery configuration. The sheath is held in a retracted delivery configuration by forming stitches of thread material around the outer circumference of the sheath, at a diameter at which the prosthesis internal of the sheath is constrained. In the embodiment of FIG. 7 of U.S. Pat. No. 6,019,785 A, a warp thread 78 is used to secure loops of the thread 75 in place around the circumference of the sheath. However, there is no indication in U.S. Pat. No. 6,019,785 A that the warp thread 78 is of any more or less flexible material than the thread 75 used to form the constraining loops. Moreover, the thread 75 in U.S. Pat. No. 6,019,785 A does not pass from a first side of overlapped edges of material, through both layers of overlapped material, cross a relatively rigid member which is disposed on a second, opposite side of the overlapped edges of material, and pass back to the first side.
WO 2008/140796 A; which is incorporated by reference in its entirety into this application, discloses a stent graft having diameter reducing ties 22, 24 formed so as to progress circumferentially around the sides of the stent graft from one end to the other. The diameter reducing ties 22, 24 are tied to release wires 18, 20, for example as illustrated in FIG. 5A of NO 2008/140796 A. The release wires 18, 20 are stitched into the material of the stent graft, to hold them in the desired position along the length of the stent graft. When the release wires 18, 20 are pulled, they release the diameter reducing ties 22, 24, allowing the stent graft to expand. The relative rigidity or flexibility of the release wires 18, 20 is not mentioned. In WO 2008/140796 A, the suture threads 22, 24 of flexible material are used to form circumferential loops, and not to form stitches along a seam at the overlapping edges of a tube of material.